1. Field of the Invention
The present invention relates to an improved process for recovering organic vapors from an organic vapor/air mixture wherein organic vapors are present in the so-called "window concentration" range, being too high for conventional carbon adsorption and too low for efficient compression/condensation recovery. More specifically, the present invention relates to separating an organic vapor and air feed stream, by use of a semipermeable membrane unit, into a vapor depleted stream that is then processed by carbon adsorption and a vapor enriched stream that is then processed by compression and condensation.
2. Description of Related Art Including
Information Disclosed under .sctn..sctn. 1.97-1.99
It is well known and a common commercial practice to employ organic solvents such as hydrocarbons, halogenated hydrocarbons, oxygenated hydrocarbons and the like in chemical processes, in the production of articles of manufacture, in the cleaning industry as well as in many other applications wherein an organic vapor/air mixture is produced as a by-product or waste stream. Historically such spent organic vapor/air streams would either be vented or flared to the atmosphere. However, contemporary emission standards, stimulated by such considerations as the "greenhouse" effect, potential depletion of stratospheric ozone as well as general health considerations and conservation of resources and basic economic considerations associated with wasting costly raw material, mandate that the organic solvent from the spent vapor/air mixtures be reclaimed.
Generally, the particular solvent and the concentration of its vapor in the air is dependent upon the particular process or application generating the spent stream. When the organic solvent is a high boiling liquid, its recovery can usually be carried out effectively by cooling to condense it to a liquid. However, when the organic solvent is a low boiling liquid there are two traditional methods for its recovery. Thus it is generally known that at high loading of organic vapor in air the organic solvent can be economically reclaimed by compression and condensation and at low loading of organic vapor in the air the organic solvent can be economically reclaimed by carbon adsorption.
The difficulty with the traditional methods is that there is a so-called "window concentration" range (typically from about 6 volume percent to 30 volume percent organic vapor) wherein neither method is practical. At too low of a concentration compression and condensation is inefficient in that greatly increased cost of compression together with reduced amount of solvent recovery and increased emission of uncondensed vapor make the process prohibitively expensive. On the other hand, at too high of a concentration the exotherm associated with carbon adsorption raises the temperature of the adsorbent bed thus reducing the adsorption efficiency and in some cases leading to the possibility of spontaneous combustion. Furthermore, high vapor concentration requires more frequent regeneration of the carbon adsorption tower thus again adding to the cost. At present, if a low boiling organic vapor and air mixture in the "window concentration" range is produced, one must further dilute the stream and then recover the organic phase by adsorption.
In U.S. Pat. No. 4,553,983 a process for recovery of organic vapor from a feed stream of air having an organic vapor content of no more than 20,000 ppm by volume (2 volume percent) is disclosed wherein a thin semipermeable membrane with a permeation selectively of at least 50 favoring the organic vapor and a permeability of at least 3.times.10.sup.-7 cm.sup.3 (STP).cm/cm.sup.2.sec.cmHg is used in combination with a partial vacuum on the permeate side to enrich the permeate with organic vapors. Similarly, in an article by D. L. Roberts et al. entitled "Recovery of Freon Gases with Silicone Rubber Membrane", Ing. Eng. Chem. Process Design Dev., 1986, 25, pp 971-973, the recovery of fluorocarbons and chlorofluorocarbons from air mixtures using a semipermeable membrane is disclosed. In U.S. Pat. Nos. 4,316,364 and 4,417,451 a refrigerant monitor system employing a membrane with a permeation selectivity favoring air relative to the refrigerant is disclosed.